U.S. Patent Application Publication No. 2014/0266068 A1 discloses a conventional battery charging apparatus for charging a lithium ion battery cell. When a voltage of the battery cell is smaller than its rated voltage value, the conventional battery charging apparatus alternates at a constant frequency and with a constant duty cycle between outputting and not outputting a charge current with a constant magnitude to charge the battery cell. Otherwise, the conventional battery charging apparatus alternates at the constant frequency and with the constant duty cycle between outputting and not outputting a charge voltage of the rated voltage value (i.e., with a constant magnitude) to charge the battery cell. The conventional battery charging apparatus causes the battery cell to degrade relatively quickly and thus have a relatively short lifetime.